The present invention relates to a method and a machine for stranding two conductors in the shape of two helices with the same winding direction or hand, offset by half the stranding length, particularly for highperformance conductors for transmitting signals in the field of communications.
It is known that the single cable used to transmit signals in the field of communications, such as for example the conventional telephone twisted pair, is generally constituted by two insulated conductors, one for the outgoing signal and one for the return signal respectively, which are stranded together, i.e., coupled one another in the form of two helices, having the same winding direction, offset by half the stranding length, in order to increase their elasticity and mechanical strength and to reduce capacity coupling.
Since the stranding length is very small, generally equal to one centimeter, and therefore one turn of the stranding machine for each stranding length is required, and since extremely large volumes of this cable are required, it is usually produced on double-twist stranding machines which have very high productivity in terms of stranding, although these machines subject the conductors to a very "rough" treatment. Stranding produced with these double-twist machines is obtained by "pinching" the conductors, which are therefore also simultaneously "twisted" about themselves.
The double-twist stranding machine in fact matches each rotation for depositing the conductor with a complete axial twist thereof; in other words, if a twisted pair produced by means of a double-twist stranding machine is observed, it can be noticed that each conductor is subjected to a full rotation about its own axis for each stranding length.
This is extremely damaging for the conductors, since the twisting that is applied to the insulating layer that covers the conductors on the one hand cracks it and separates it from the copper and on the other hand subjects the core to pinching and to irregular reductions in cross-section in the points where it yields.
This fact was acceptable in the past in view of the low passband that was required, but it is becoming increasingly intolerable as the required performance of the cable increases, requiring perfect insulations and absolute constancy of the properties of the conductor along the entire path of the signal.
In order to obviate the problem of conductor twisting, stranding machines are currently manufactured which provide preventive partial detwisting, i.e., in which the individual conductor, before being paired, is subjected to a 50-60% twisting which is opposite in sign or hand (detwisting) to the twisting that it will undergo during the subsequent stranding operation, so as to leave, at the end of the process, a conductor with reduced residual twisting.
This refinement provides no substantial advantages with respect to the previous method, since actually it subjects the cable to two processes and therefore to twice as much damage by means of its two inverse and opposite twisting operations, with the effect of separating even more the insulation from the copper, degrading the performance of the cable even more than the preceding double-twist process. The degradation of the performance of the cable does not depend on the absolute residual twisting but on the absolute treatment to which it has been subjected. In practice, although a 180.degree. twisting of the conductor about its own axis and a 360.degree. countertwisting at each stranding length ultimately leave a conductor with an absolute twist of only 180.degree. (360.degree.-180.degree.), it worsens the performance of the conductor as if it had undergone a total of 540.degree. of twisting.